Digital optical spatial switches (DOS) are well known in the art. One class of such switches is the 1×2 digital optical switch known as a “Y-branch digital optical switch” (DOS) wherein light input into the “base” or trunk of the Y is directed through one or the other of the output branches by virtue of changes effected in the refractive index of one or both of the output branches. The switch can also be operated in reverse, whereby one or the other “upper branches” of the Y can be selected as an input channel with the base of the Y being the output channel. Y-branches are a fundamental building block of optical circuitry, and may be employed singly, or in various combinations to form more complex switching and coupling devices.
The Y-branch DOS has received wide commercial acceptance primarily because of its robustness to variations in critical parameters such as electrical power applied, polarization, wavelength, temperature, and to a large extent, even device geometrical variations. Typically a Y-branch DOS is designed such that two waveguide branches intersect to define a Y-shaped structure with a very small angle at the intersection of the branches. The composition of the waveguide structure may include a wide variety of materials such as lithium niobate, semiconductors, silica, or polymers. A Y-branch DOS performs its switching function by adiabatically changing (i.e. slowly varying, as opposed to abruptly altering) the light propagation direction in one of the output waveguides.
Specifically, switching in a Y-branch DOS is achieved by forcing a refractive index change in one waveguide branch with respect to the other. Of particular significance among the characteristics of a Y-branch DOS is its step-like response. When a Y-branch DOS operates above a switching threshold, variations in polarization and wavelength do not impact significantly the switching capacity of the Y-branch DOS.
One persistent problem presented to the designer of optical communications systems is related to the security of the data transmission in the event of an electrical power failure. Numerous optical communications components are electro-optic in nature. When an electrical power failure occurs optical data signals could potentially be misdirected. It is important to provide a fail-safe mechanism by which secure optical transmissions are not inadvertently diverted to the wrong recipient during a power failure. This can be achieved by placing in line a normally dark digital optical switch (ND-DOS), or its variant, a normally dark variable optical attenuator (ND-VOA). The ND feature also helps avoid high optical power, which could destroy photodetectors. The ND feature also permits optical power to be shut off without the expenditure of electrical power.
The ND feature is readily available in optomechanical switches and microelectromechanical system switches/VOA's). The challenge of attaining desired levels of signal attenuation in planar light wave circuits (PLCs) is particularly daunting because of the close proximity of optical circuit elements and the high incidence of cross-talk. Signal attenuation of ≧40 dB is needed for the most critical applications. One approach taught in the art to providing a ND feature in PLCs is to use an interferometric design, such as the well-known Mach-Zehnder Interferometer (MZI) as in Jinguji et al, IEEE J Lightwave Technology 14 (10), 2301–2310 (1996). However, available devices exhibit ˜20 dB attenuation with high wavelength dependence. While these devices are suitable for some applications, improvements are needed.
Switching in Y-branches is accomplished by effecting a change in refractive index in one of the branches. Several methods are known in the art for effecting the desired change in refractive index. These include the electro-optic effect, the stress-optic effect and the thermo-optic effect. In a typical Y-branch thermo-optic DOS known in the art, the two upper branches of the “Y” are provided with a heating means, typically a thin layer of metal deposited thereupon, which heating means when activated induces a shift in the refractive index of the corresponding branch, thereby effecting a coupling of power input to the base of the “Y” to one or the other branches. By turning on the heating of one branch and turning off the heating of the other branch, switching of incoming optical signals can be effected. By heating both branches at once a gradual signal attenuation may be effected.
Both polymeric and glass Y-branches are known. Because of the much larger temperature dependence of the refractive index of polymers, polymers are preferred for use in thermo-optic digital optical switches.
Hida et al, IEEE Photonics Technology Letters 5 (7), 782–784 (1997), disclose polymeric 2×2 thermo-optic switches consisting of two coupled Y-branches fabricated from deuterated and fluoro-deuterated methacrylate polymers. The method of fabrication involves spin coating polymer solutions onto a silicon substrate followed by forming the Y-shaped components by conventional photolithography, the core ridges being subsequently formed by reactive ion etching. Chromium thin-film strip heaters were formed on the upper Y-branches by electron beam evaporation and wet etching. Separation of the arms was 250 micrometers. The Cr heater strips were 5 mm long and 50 micrometers wide.
Eldada et al, Proc. SPIE, vol. 3950, pp. 78–89 (2000), discloses 1×2 optical switches fabricated from polymeric materials, which Y-branches exhibit 0.1 dB insertion loss for vertex angles of less than 2°. The direct photolithographic fabrication method using halogenated acrylates as practiced therein is disclosed to enable sharp profiles of the components and the removal of residue even at the vertex of relatively small angle Y-branches.
Lackritz et al, U.S. Pat. No. 6,236,774B1, discloses thermo-optic switches employing cross-linked polymeric waveguides operated above Tg. Disclosed are metallic heaters substantially rectangular in shape disposed upon a polymeric optical waveguide surface, the long side of said rectangular heater being positioned at a slight angle to the direction of propagation in the waveguide. Said heaters are positioned to be in uniform thermal contact with waveguide material over the entire area of the heater. It is disclosed that the temperature, and therefore the refractive index, of the polymer waveguide material will depend upon the distance of any point therein from the heater, those regions closest to the heater experiencing greater temperature than those further away.
Lee et al, U.S. Pat. No. 5,623,566, is drawn to thermally induced guides in silicon optical benches. Disclosed in FIG. 2 thereof is the temperature profile through the various optical materials employed therein as a result of localized heating applied thereto.
Moosburger et al, Proc. 21st Eur. Conf. on Opt. Comm, pp. 1063–1067 (1995) disclose Y-branches with “near perfect” vertices having an angle of 0.12° fabricated from silica-clad polymeric waveguides having cores of ca. 9 micrometers. The upper branches of the Y were coated with Ti thin film heaters. 27 dB cross-talk suppression was achieved between output branches with heater power of ca. 180 mW.
Diemeer, Optical Materials 9, 192–200 (1998) provides a thoroughgoing analysis of the thermal transport and physical aspects of thermo-optic switching in polymeric vs. silica thermo-optic digital optical switches. For polymers in general, and polycarbonate and polymethylmethacrylate in particular, it is shown that switching power lies in the range of 50–100 mW, and that a temperature rise of ca. 10° C. in the waveguide core is necessary to achieve a minimum refractive index difference of ca. 0.001.
Lee et al., IEEE Photonics Technology Letters, 11 (5), 1999, pp. 590–592 discloses a polymeric tunable optical attenuator fabricated using asymmetric DOS/VOA waveguides integrated with an optical monitoring tap, where the attenuation level is minimal when no electrical power is applied.
Tamir et al., Guided Wave Optoelectronics, Springer-Verlag, 1988 pp. 121–125 discloses asymmetric Y-branch waveguides for use as mode splitters and power dividers.